U.S. patent number 6,868,318 [Application Number 10/686,174] was granted by the patent office on 2005-03-15 for method for adjusting battery power limits in a hybrid electric vehicle to provide consistent launch characteristics.
This patent grant is currently assigned to General Motors Corporation. Invention is credited to William R. Cawthorne, Gregory A. Hubbard.
United States Patent |
6,868,318 |
Cawthorne , et al. |
March 15, 2005 |
Method for adjusting battery power limits in a hybrid electric
vehicle to provide consistent launch characteristics
Abstract
A method is disclosed for improving the performance of an energy
storage system that incorporates a high density electrical energy
storage device, such a battery or ultracapacitor. The method may be
implemented in an energy storage system of a hybrid electric
vehicle (HEV) as a computer control algorithm for controlling the
discharge power limits of an energy storage device, such as a
battery. The method allows the discharge power limits of the
battery to be temporarily expanded under vehicle launch conditions
where the power demands are high, thereby making additional stored
energy available for use under such conditions by improving battery
utilization and providing more consistent vehicle launch
characteristics than would otherwise be available.
Inventors: |
Cawthorne; William R. (Milford,
MI), Hubbard; Gregory A. (Brighton, MI) |
Assignee: |
General Motors Corporation
(Detroit, MI)
|
Family
ID: |
34274798 |
Appl.
No.: |
10/686,174 |
Filed: |
October 14, 2003 |
Current U.S.
Class: |
701/22; 307/10.1;
318/139; 320/134 |
Current CPC
Class: |
B60W
10/08 (20130101); B60W 20/13 (20160101); B60L
58/12 (20190201); B60W 10/26 (20130101); B60W
30/18027 (20130101); B60L 50/40 (20190201); B60W
20/19 (20160101); B60L 50/16 (20190201); Y02T
10/70 (20130101); B60W 2720/106 (20130101); B60W
20/00 (20130101); B60W 2710/086 (20130101); B60W
2510/244 (20130101); B60W 2710/083 (20130101); Y02T
10/7072 (20130101); B60W 2710/248 (20130101); B60W
2710/244 (20130101) |
Current International
Class: |
B60K
6/04 (20060101); B60K 6/00 (20060101); H02J
007/00 (); B60K 031/02 () |
Field of
Search: |
;307/64,10.8,10.1
;320/162,128,126,125,136,119,108,104,150 ;180/65.8,65.2,65.3 ;280/1
;290/16,27 ;318/139,432,368,143 ;370/127 ;701/22,102,110,114 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Black; Thomas G.
Assistant Examiner: To; Tuan C
Attorney, Agent or Firm: Hodges; Leslie C.
Claims
What is claimed is:
1. A method of implementing a wide limit mode (WLM) of operation in
a vehicle comprising an energy storage system having a rechargeable
battery, the battery having a plurality of monitored battery
parameters, a discharge power limit and a closed-loop controller,
the controller having a timer and a counter that are adapted to
count time intervals associated with the implementation of the WLM
by incrementing a count when the WLM is active and decrementing the
count or maintaining a zero count when the WLM is not active,
comprising the steps of: (1) determining whether the WLM is active;
(2) setting a WLM discharge power limit when the WLM is active that
is greater than the discharge power limit; and (3) establishing a
duty cycle for the WLM using the timer and counter, wherein the
duty cycle comprises a maximum time interval during which the WLM
discharge power is available for use by the vehicle and a minimum
time interval during which the WLM discharge power is not available
for use by the vehicle.
2. The method of claim 1, wherein determining whether WLM is active
in step (1) is performed using a vehicle speed input and a vehicle
torque command input.
3. The method of claim 2, comprising the further steps of: (1a)
determining a WLM output speed ratio (R.sub.N) that characterizes
the WLM as a function of the vehicle output speed input; (1b)
determining a WLM output torque ratio (R.sub.T) that characterizes
the WLM as a function of the vehicle output torque command input;
(1c) multiplying R.sub.N and R.sub.T to obtain a WLM ratio; wherein
the WLM ratio (R.sub.WLM) has a value which determines whether the
WLM is active or not.
4. The method of claim 3, wherein R.sub.N comprises a value in the
range -1<R.sub.N <1, R.sub.T comprises a value in the range
-1<R.sub.T <1, and wherein WLM is active for values of
R.sub.WLM, where 0<R.sub.WLM <1, and WLM is not active for
all other values of R.sub.WLM.
5. The method of claim 4, wherein the value of R.sub.N as a
function of vehicle speed and the value of R.sub.T as a function of
the torque command input are determined from a look-up table.
6. The method of claim 1, wherein setting the WLM discharge power
limit of step (2) comprises shifting the discharge power limit by
an amount that is determined by imputing a change in at least one
parametric battery discharge power limit as a function of at least
one battery parameter.
7. The method of claim 6, comprising the further steps of: (2a)
shifting the monitored value of at least one of the monitored
battery power parameters (B.sub.1) to a lower value (B.sub.2); and
(2b) shifting the parametric battery power discharge limit
(P.sub.1) associated with B.sub.1 to an increased value associated
with a shifted parametric battery power discharge limit (P.sub.2)
in accordance with a parametric relationship between the at least
one parameter and the parametric battery power discharge limit.
8. The method of claim 7, wherein the amount by which the at least
one parametric discharge power limit is increased and the value of
the WLM discharge power limit are directly proportional to the
value of R.sub.WLM.
9. The method of claim 8, wherein the plurality of battery power
parameters are selected from the group consisting of a battery
temperature, battery state of charge and battery amp-hour
throughput.
10. The method of claim 1, wherein step (3) comprises the further
steps of: (3a) incrementing the value of the count (C) by an
increment associated with a control loop; (3b) determining the
value of C and whether WLM is active, wherein if C is less than a
WLM maximum count limit (C.sub.M) and WLM is active, returning to
step (1), and wherein if C is equal to C.sub.M or WLM is not
active, proceeding to step (3c); (3c) setting the discharge power
limit; (3d) decrementing the value of C by a decrement associated
with a control loop; (3e) determining the value of C and whether
WLM is active; wherein if the count is greater than a WLM dead band
limit (C.sub.DB) and WLM is active or not active, returning to step
(3c); and wherein if the count is less than or equal to C.sub.DB
and greater than zero and WLM is not active, returning to step
(3c); and wherein if the count is less than or equal to C.sub.DB
and greater than or equal to zero and WLM is active, returning to
step (1); and wherein if the count is equal to zero and WLM is
inactive, proceeding to step (3f); (3f) holding the count at zero
until WLM is active, and then returning to step (1).
11. A control algorithm for implementing a wide limit mode (WLM) of
operation in a vehicle comprising an energy storage system having a
rechargeable battery, the battery having at least one monitored
battery power parameter, at least one parametric discharge power
limit and a closed-loop controller operating through the execution
of a plurality of control loops and adapted to determine whether
the WLM is active or not active, the controller having a timer and
a counter that are adapted to count time intervals associated with
the implementation of the WLM by incrementing a count when the WLM
is active and decrementing the count or maintaining a zero count
when the WLM is not active; comprising the steps of: (1)
determining a WLM ratio using a vehicle speed input and a vehicle
torque command input; (2) initializing the timer, counter and a
control loop, wherein the WLM ratio is set to zero for the first
control loop; (3) determining whether the WLM is active and whether
the timer is expired, and if the WLM is active and the timer is not
expired, proceeding to step (4), otherwise, proceeding to step (5);
(4) incrementing the timer and counter and proceeding to step (8);
(5) setting the WLM ratio to zero; (6) decrementing the timer and
counter and proceeding to step (7); (7) setting a lower limit for
the counter and proceeding to step (8); (8) determining the amount
of a WLM discharge power limit shift that is based on the WLM
ratio; (9) applying the WLM discharge power limit shift to at least
one parametric discharge power limit; and (10) determining whether
the control loop is the last control loop, wherein if the control
loop is the final control loop, and if yes, ending the algorithm,
and if no, iterating the algorithm for another control loop by
returning to step (1).
12. The method of claim 11, wherein calculating a WLM ratio using a
vehicle speed input and a vehicle torque command input comprises
the further steps of: (1a) determining a WLM speed ratio (R.sub.N)
that characterizes the WLM as a function of the vehicle speed
input; (1b) determining a WLM torque ratio (R.sub.T) that
characterizes the WLM as a function of the vehicle torque command
input; (1c) multiplying R.sub.N and R.sub.T to obtain a WLM ratio
(R.sub.WLM); wherein the WLM ratio has a value which determines
whether the WLM is active.
13. The method of claim 12, wherein R.sub.N comprises a value in
the range -1<R.sub.N <1, R.sub.T comprises a value in the
range -1<R.sub.T <1, and wherein WLM is active for values of
R.sub.WLM, where 0<R.sub.WLM <1, and WLM is not active for
all other values of R.sub.WLM.
14. The method of claim 13, wherein the value of R.sub.N as a
function of the vehicle speed and the value of R.sub.T as a
function of the torque command input are determined from a look-up
table.
15. The method of claim 11, wherein the step of determining the
amount of a WLM discharge power limit shift comprises imputing a
change in at least one parametric battery discharge power limit as
a function of at least one monitored battery power parameter.
16. The method of claim 15, comprising the further steps of: (8a)
shifting the monitored value of at least one of the monitored
battery power parameters (B.sub.1) to a lower value (B.sub.2); and
(8b) shifting the parametric battery power discharge limit
(P.sub.1) associated with B.sub.1 to an increased value associated
with a shifted parametric battery power discharge limit (P.sub.2)
in accordance with a parametric relationship between the at least
one parameter and the parametric battery power discharge limit.
17. The method of claim 16, wherein the amount by which the at
least one parametric discharge power limit is increased and the
value of the WLM discharge power limit are directly proportional to
the value of R.sub.WLM.
18. The method of claim 17, wherein the plurality of battery power
parameters are selected from the group consisting of a battery
temperature, battery state of charge and battery amp-hour
throughput.
Description
TECHNICAL FIELD
This invention comprises a method for controlling the energy
storage system (ESS) in a hybrid electric vehicle (HEV). More
particularly, the method comprises a computer control algorithm for
determining the discharge limits for the battery in an HEV, such
that it is protected from damage and yet is capable of maximum
available utilization. Most specifically, the method comprises a
computer control algorithm for expanding the discharge power limits
of the battery of an HEV under launch conditions while also
maintaining the overall integrity of the discharge power protection
limits.
BACKGROUND OF THE INVENTION
An HEV is a vehicle that has a propulsion system that consists of
at least one electric motor or electric machine in combination with
at least one other power source. Typically, the other power source
is a gasoline or diesel engine. There are various types of HEVs
depending on how the electric motor(s) and other power source(s)
are combined with one another in order to provide propulsion for
the vehicle, including series, parallel and compound HEVs.
Various hybrid powertrain architectures are known for managing the
input and output torques of various propulsion systems in HEVs,
most commonly internal combustion engines and electric machines.
Series hybrid architectures are generally characterized by an
internal combustion engine driving an electric generator which in
turn provides electrical power to an electric drivetrain and to an
energy storage system, comprising a battery pack. The internal
combustion engine in a series HEV is not directly mechanically
coupled to the drivetrain. The electric generator may also operate
in a motoring mode to provide a starting function to the internal
combustion engine, and the electric drivetrain may recapture
vehicle braking energy by also operating in a generator mode to
recharge the battery pack.
Parallel HEV architectures are generally characterized by an
internal combustion engine and an electric motor which both have a
direct mechanical coupling to the drivetrain. The drivetrain
conventionally includes a shifting transmission to provide the
necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide
for continuously variable speed ratios by combining features from
both series and parallel HEV powertrain architectures. EVTs are
operable with a direct mechanical path between an internal
combustion engine and a final drive unit thus enabling high
transmission efficiency and application of lower cost and less
massive motor hardware. EVTs are also operable with engine
operation mechanically independent from the final drive or in
various mechanical/electrical split contributions (i.e. input
split, output split and compound split configurations) thereby
enabling high-torque continuously variable speed ratios,
electrically dominated launches, regenerative braking, engine off
idling, and two-mode operation.
As noted, such complex EVT HEVs utilize one or more electric
machines and require advanced energy transmission, conversion and
storage systems to supply electrical energy to and receive and
store electrical energy from these machines, and would typically
comprise, for example, at least one electric machine, power
inverter module, power bus, energy storage device, such as a
battery, as well as various control electronics, control algorithms
and other associated items. The energy storage system (ESS) may
comprise any suitable energy storage system that is adapted for
high-density energy storage, including a battery, ultracapacitor,
or other high-density energy storage device. As used herein,
reference to a battery includes not only a single battery, also
includes any combination of single or multiple batteries, or cells
thereof, into a battery pack or array, or a plurality of battery
packs or arrays. This invention is particularly suitable for use in
a parallel array of battery packs, each of which comprised a
plurality of batteries. As used herein, the term battery generally
refers to any secondary or rechargeable battery, but those
comprising lead/acid, nickel/metal hydride (Ni/MH, or Li/ion or
polymer cells are preferred.
Given the dynamics associated with operation of an HEV,
particularly the constant flow of energy into and out of the energy
storage device, the battery plays a critical role in the operation
of these vehicles. The critical role of the battery in these
vehicles imposes a number of requirements on the battery
performance, including both operational and service life
requirements.
Significant attention has been given to maintaining the operational
performance of batteries used in HEV applications. Particular
attention has been given to various aspects of maintaining the
battery pack state of charge (SOC). The SOC is defined generally as
the ratio of the residual charge in a battery relative to its full
charge capacity. Various hardware and software control strategies
have been adjusted for determining and maintaining the SOC of the
battery.
While understanding and maintaining the SOC of the battery is
critical to its performance in HEV applications, it is not the only
important characteristic of the battery. Another critical
characteristic of batteries used in HEV applications is the useful
life of the battery or battery pack. For example, it is known that
secondary batteries, such as Ni-MH batteries, have limited amp-hour
throughput that defines their useful service life. The anp-hour
throughput or capacity of the battery is the integral of the energy
flowing through the battery as a function of time as it is
constantly charged and discharged in service.
While it is critical to manage various aspects of the ESS of an HEV
as described above, it is also necessary to ensure certain aspects
of vehicle performance, such as the vehicle launch characteristics.
Vehicle launches are generally associated with starting the motion
of the vehicle from a stop, and may be characterized by the speed
of the vehicle and its required torque output at any given point
during operation of vehicle (i.e., no or low speed and relatively
high torque). However, launch conditions may also exist during
other periods of vehicle operation, such as acceleration from a
low-speed interval, or seeking to maintain or increase speed while
negotiating an incline. Therefore, it is desirable to develop
control algorithms for vehicle operation which ensure the
management and protection of the ESS, particularly the battery,
while at the same time ensuring that the ESS, including the
battery, may be fully utilized to ensure optimum vehicle
performance under launch conditions.
SUMMARY OF THE INVENTION
The invention may be described generally as a method of
implementing a wide limit mode (WLM) of operation in a vehicle
comprising an energy storage system having a rechargeable battery,
the battery having a plurality of monitored battery parameters, a
discharge power limit and a closed-loop controller, the controller
having a timer and a counter that are adapted to count time
intervals associated with the implementation of the WLM by
incrementing a count when the WLM is active and decrementing the
count or maintaining a zero count when the WLM is not active,
comprising the steps of: (1) determining whether the WLM is active;
(2) setting a WLM discharge power limit when the WLM is active that
is greater than the discharge power limit; and (3) establishing a
duty cycle for the WLM using the timer and counter, wherein the
duty cycle comprises a maximum time interval during which the
increased discharge power associated with the WLM is available for
use by the vehicle and a minimum time interval during which the
increased discharge power associated with the WLM is not available
for use by the vehicle.
The method is preferably implemented as a computer control
algorithm in a closed loop controller that is adapted to control
the battery discharge power limits. The method is used to
temporarily extend the battery power limits to less restrictive
values during vehicle launch maneuvers and thereby allow the HEV to
provide consistent launch performance using the batteries in a
number of operational situations that would otherwise result in
limited battery availability and reduced vehicle performance while
also maintaining the maximum battery discharge power limits and the
protections that they afford the battery.
The invention overcomes the deficiencies of the prior art by
providing a means for expanding the battery discharge power limits
during launch maneuvers such that consistent vehicle launch
performance is maintained or improved.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the
detailed description given here below, the appended claims, and the
accompanying drawings in which:
FIG. 1 is a functional block diagram generally illustrating of the
steps of the method of the present invention.
FIG. 2 illustrates the propulsion modes for the various
combinations of input speed (No) and input torque command (To),
including those for which the WLM is active.
FIG. 3 is a functional block diagram generally illustrating the
steps of block 20 of FIG. 1.
FIG. 4 is a functional block diagram generally illustrating the
steps of block 32 of FIG. 3.
FIG. 5 is a functional block diagram generally illustrating the
steps of block 26 of FIG. 3.
FIG. 6 is a graph illustrating a comparison of the WLM status,
timer status and timer count for exemplary values of the WLM status
as a function of time.
FIG. 7 is a schematic representation of a plurality of discharge
power related limits, illustrating a WLM shift associated with
each.
FIG. 8 is a graph illustrating the WLM shift as a function of the
WLM ratio.
DESCRIPTION OF THE PREFERRED EMBODIMENT
This invention provides a method for adjusting battery discharge
power limits. More specifically, it may be used as a control
algorithm for implementing a wide limit mode (WLM) of operation in
an HEV, of the type described in commonly assigned, co-pending U.S.
patent application Ser. Nos. 10/686,034 filed Oct. 14, 2003 and
10/686,508 filed Oct. 14, 2003 which awe herein incorporated by
reference in their entirety, a compound split EVT HEV is disclosed
which has characteristics of both a series and parallel HEV which
generally comprise at least one drive motor, such as an internal
combustion engine, at least one electric machine that is adapted to
both provide propulsion to the vehicle and generate electric power
for storage on the vehicle, and an ESS which typically comprises a
rechargeable or secondary battery, as described herein. The WLM may
be used to provide consistent vehicle launch characteristics.
Vehicle launch conditions are defined broadly as conditions under
which it is desirable to charge the battery to provide vehicle
propulsion, generally where the vehicle output speed is low and the
desired output torque is high, such as acceleration from a stop,
acceleration up an incline, and other operating conditions where it
is desirable to discharge the ESS for vehicle propulsion. Launch
conditions may be defined by a range of vehicle speeds and a range
of desired vehicle output torques, or output torques, associated
with those vehicle speeds. Due to the dynamic charge/discharge
conditions experienced by the ESS in an HEV, and the desire to
monitor and control various battery parameters associated with
these conditions, it is frequently the case that the discharge
power of an ESS is limited by control actions taken due to ESS
parameters. In the case of an ESS comprising a rechargeable
battery, these may include battery parameters such as the state of
charge (SOC), temperature and energy throughput. A method of
determining parametric battery power limits for an ESS of an HEV
which takes into account the battery SOC, temperature and amp-hour
throughput is described in commonly assigned, copending U.S.
provisional patent application Ser. No. 60/511,456 filed Oct. 15,
2003 and U.S. patent application Ser. No. 10/686,180 filed Oct. 14,
2003, which are both herein incorporated by reference in their
entirety. Consequently, when the maximum battery discharge power is
limited or reduced in conjunction with such control actions, the
vehicle launch performance is as a result reduced as compared to
situations where the batteries are capable of fill utilization and
provision of the maximum battery diverge power. This invention
provides a method for temporarily widening the battery discharge
power limits such that more battery power can be utilized for short
times during vehicle launch to provide consistent vehicle
performance without damaging the battery.
The invention utilizes the output speed of the vehicle and the
commanded output torque at that speed to determine the status of
the vehicle and whether a launch condition exists and whether the
WLM is active or not active. The algorithm determines the vehicle
is in a launch condition based on a comparison of the output speed
of the vehicle and the commanded output torque. According to the
method of the invention, the WLM is activated for certain
combinations when the output speed is low and the commanded output
torque is high, and also takes into account not only the magnitude
of these values, but their direction, as illustrated in FIG. 2. The
WLM is not activated for other combinations of vehicle speed. When
these conditions are met, the algorithm determines that the system
is in a launch condition and the WLM allows for expanded battery
discharge power limits in order to provide the desired vehicle
performance under launch conditions.
The method of the invention may be implemented in any suitable
closed-loop controller within the vehicle. It is preferably
implemented within the controller associated with the performance
of the ESS, but may be implemented in any suitable closed loop
controller within the vehicle that is in signal communication with
the ESS.
The method 10 of the invention is illustrated generally in FIG. 1,
and may be described as a control algorithm 10 for implementing a
wide limit mode (WLM) of operation in a vehicle comprising an
energy storage system having a rechargeable battery, the battery
having at least one monitored battery power parameter, at least one
parametric discharge power limit and a closed-loop controller
operating through the execution of a plurality of control loops and
adapted to determine whether the WLM is active or not active, the
controller having a timer and a counter that are adapted to count
time intervals associated with the implementation of the WLM by
incrementing a count when the WLM is active and decrementing the
count or maintaining a zero count when the WLM is not active;
comprising the steps of: (1) determining a WLM ratio 20 using a
vehicle speed input and a vehicle torque command input; (2)
initializing the timer, counter and a control loop 40, wherein the
WLM ratio is set to zero for the first control loop (3) determining
whether the WLM is active and whether the timer is expired 50, and
if the WLM is active and the timer is not expired, proceeding to
step (4), otherwise, proceeding to step (5); (4) incrementing the
timer and counter 70 and proceeding to step (8); (5) setting the
WLM ratio to zero 80; (6) decrementing the timer and counter 90 and
proceeding to step (7); (7) setting a lower limit for the counter
100 and proceeding to step (8); (8) determining the amount of a WLM
discharge power limit shift that is based on the WLM ratio; (9)
applying the WLM discharge power limit shift to at least one of the
parametric discharge power limits; and (10) determining whether the
control loop is the last control loop, wherein if the control loop
is the final control loop, and if yes, ending the algorithm, and if
no, iterating the algorithm for another control loop by returning
to step (1). These steps are described further below. The
determination of control loop status and whether the WLM is to be
active or not may be divided among a plurality of system
controllers.
Referring to FIGS. 3-5, the step of determining a WLM ratio 20
using a vehicle output speed input and a vehicle output torque
command input is described in greater detail. The WLM ratio is a
simplified means of simultaneously characterizing both the
magnitude and direction of both a vehicle speed input and a vehicle
torque command input over a range of values of these quantities, so
that decisions can be made about activation and deactivation of the
WLM as a function of these vehicle parameters. The vehicle output
speed input is the actual vehicle speed and may be obtained by any
of a number of known methods, such as utilizing the output of a
speedometer, tachometer or other rotational motion or rate sensing
means associated with the vehicle drivetrain. The output speed
input is typically represented as either a positive or negative
value depending on the direction of vehicle motion (i.e. forward or
reverse motion), with the magnitude of the input indicating the
magnitude of the speed. Means for obtaining a vehicle speed input
are well-known. The vehicle output torque command input is a
calculated value that may be obtained from any vehicle controller
that is adapted to control the vehicle powertrain performance,
including the torque output of the vehicle powertrain.
As shown in FIG. 3, the step of determining WLM ratio 20 using
vehicle output speed 22 input and vehicle output torque command 24
input may be performed by the further steps of (1a) determining a
WLM output speed ratio (R.sub.N) 26 that characterizes the WLM as a
function of the vehicle output speed 22 input; (1b) determining a
WLM output torque ratio (R.sub.T) 32 that characterizes the WLM as
a function of vehicle output torque command 24 input; and (1c)
multiplying R.sub.N 26 and R.sub.T 32 to obtain WLM ratio
(R.sub.WLM) 38; wherein WLM ratio 38 has a value which determines
whether the WLM is active.
The value of R.sub.T 34 is determined using lookup table 32 with
the desired torque output (T.sub.O) 24 of the vehicle as an input
and R.sub.T 36 as the output, as shown in FIG. 4. The axes of the
lookup table are established as calibratable values in the
controller such that the values can be tuned to produce the desired
vehicle response under launch conditions. It is believed that the
preferred characteristic response of R.sub.T 34 as a function of
the vehicle output torque command 24 is illustrated in FIG. 4. In
the implementation shown in FIG. 4, negative values of T.sub.O
correspond to output command torques 24 associated with either
forward regeneration or reverse propulsion, depending on the
direction of vehicle motion, as illustrated in FIG. 2. Likewise,
positive values of T.sub.O correspond to output command torques 24
associated with either forward propulsion or reverse regeneration,
depending on the direction of vehicle motion, as illustrated in
FIG. 2. As shown in FIG. 4, moving away from the origin in the
direction of negative torque, R.sub.T 34 begins at zero and remains
there until a negative torque transition threshold (T.sub.N) is
reached, whereupon R.sub.T 34 transitions at a rapid rate to a
value of -1, and thereafter remains at -1. Therefore, R.sub.T
comprises a value in the range -1<R.sub.T <1. As also shown
in FIG. 4, moving away from the origin in the direction of positive
torque, R.sub.T 34 also begins at zero and remains there until a
positive torque transition threshold (T.sub.P) is reached,
whereupon R.sub.T 34 transitions at a rapid rate to a value of 1,
and thereafter remains at 1. The values of T.sub.N and T.sub.P may
be selected to tune the WLM with regard to the vehicle output
torque commands that define a launch condition, and may also be
varied from one vehicle type to another so as to differentiate the
definition of a launch condition, and hence, vehicle launch
performance by vehicle type. For a relatively large vehicle, such
as a bus, T.sub.N was selected to be about -200 N-m, and T.sub.P
was selected to be about 300 N-m. The values of these transitions
and the shape of this curve may be symmetric or asymmetric,
depending on the desired launch response as a function of output
torque command. The object of the selection of the values of
R.sub.T and the characteristics shape of the function described is
to allow WLM to be active only when the vehicle is operating at
higher torque levels. WLM is intended to extend the battery limits
only when the vehicle is in a launch condition. A launch maneuver
that would require extended battery power for consistent driver
feel would typically be a high output torque maneuver at relatively
low vehicle speeds, such as a rapid acceleration from a stop or low
vehicle speed. Thus, the function of the output torque command
based component of the WLM ratio is primarily binary in nature and
is used to signify whether the output torque command is
representative of a high torque launch where activation of the WLM
would be required.
Similarly, the value of R.sub.N 28 is determined using lookup table
26 with N.sub.O 22 of the vehicle as an input and R.sub.N 28 as the
output, as shown in FIG. 5. The axes of the lookup table are
established as calibratable values in the controller such that the
values can be tuned to produce the desired vehicle response under
launch conditions. It is believed that the preferred characteristic
response of R.sub.N 28 as a function of the vehicle output speed 22
is illustrated in FIG. 5. In the implementation shown in FIG. 5,
negative values of N.sub.O correspond to output speeds 22
associated with either reverse propulsion or reverse regeneration,
depending on the direction of the output torque command, as
illustrated in FIG. 2. Likewise, positive values of N.sub.O
correspond to output speeds 22 associated with either forward
propulsion or forward regeneration, depending on the direction of
output torque command, as illustrated in FIG. 2. As shown in FIG.
5, moving away from the origin in the direction of negative speed,
R.sub.N 28 begins at zero which is a transition point and decreases
at a relatively rapid rate to a value of -1 land remains there
until a negative speed transition threshold (N.sub.N) is reached,
whereupon R.sub.N 28 again transitions to a value of 0, and
thereafter remains at 0. As also shown in FIG. 5, moving away from
the origin in the direction of positive speed, R.sub.N 28 begins at
zero which is a transition point and increases at a relatively
rapid rate to a value of 1 and remains there until a positive speed
transition threshold (N.sub.P) is reached, whereupon R.sub.N 28
again transitions to a value of 0, and thereafter remains at 0.
Therefore, R.sub.N comprises a value in the range -1<R.sub.N
<1. The values of N.sub.N and N.sub.P and the rates of
transition surrounding them may be selected to tune the WLM with
regard to the vehicle output speeds that define a launch condition,
and may also be varied from one vehicle type to another so as to
differentiate the definition of a launch condition, and hence,
vehicle launch performance by vehicle type. For a relatively large
vehicle, such as a bus, N.sub.N was selected to be about -8 Kph
with the transition complete to a value of 0 at a speed of about
-11 Kph, and N.sub.P was selected to be about 8 Kph with the
transition complete to a value of 0 at a speed of about 11 Kph. The
values of these transitions and the shape of this curve may be
symmetric or asymmetric, depending on the desired launch response
as a function of output speed. The object of the selection of the
values of R.sub.N and the characteristics shape of the function
described is also to allow WLM to be active only when the vehicle
is operating at relatively low vehicle speeds, and to ramp out
smoothly as speed increases. A launch maneuver that would require
extended battery power for consistent driver feel would typically
be a low speed, high output torque maneuver, such as a rapid
acceleration from a low speed. Thus, the function of the output
speed based component of the WLM ratio is also primarily binary in
nature and is used to signify whether the output speed is
representative of a low speed launch where activation of the WLM
would be required.
Referring again to FIG. 1, step (2) of control algorithm 10
comprises initializing the timer, counter and a control loop 40,
wherein the WLM ratio is set to zero for the first control loop.
The WLM timer and counter are preferably incorporated into the
closed-loop controller that is used to implement the WLM. The WLM
timer is implemented using a counter which incorporates hysteresis
on the counter triggering.
Referring again to FIG. 1, step (3) of control algorithm 10
comprises determining whether the WLM is active and whether the
timer is expired 50, and if the WLM is active and the timer is not
expired, proceeding to step (4), otherwise, proceeding to step (5).
The WLM is active in a given control loop if the composite WLM
ratio as calculated in the previous control loop and described
below is greater than zero. If the WLM ratio is zero, either one or
both of the output torque based ratio and/or the output speed based
ratio was such that the system should not be in WLM. During the
first loop, the WLM ratio is initialized to zero. Referring to FIG.
2, the lookup tables for R.sub.N and R.sub.T have positive values
for positive speed and torque and negative values for negative
speed and torque, thereby permitting R.sub.WLM to be positive and
WLM to be active and operate when the vehicle is in the reverse
speed range, but not operate when the vehicle is in regeneration
mode (positive output torque command). In forward propulsion mode,
the output torque and speed would both be positive, as would
R.sub.N and R.sub.T, so the composite WLM ratio would also be
positive. In forward regeneration, R.sub.N would be positive, but
R.sub.T would be negative, so R.sub.WLM would be negative and WLM
would not be active. Logic is included to limit R.sub.WLM to
positive values, so any negative composite ratios would be set to
zero, thereby inactivating the WLM. If the WLM is active and the
timer is not expired, control algorithm 10 proceeds to step (4),
otherwise, control algorithm 10 proceeds to step (5).
Step (4) of control algorithm is comprises the step of incrementing
the timer and counter 70 and proceeding to step (8). Alternately,
steps (5)-(8) of control algorithm comprise step (5) of setting the
WLM ratio to zero 80; step (6) of decrementing the timer and
counter 90 and step (7) of setting a lower limit for the counter
100 and proceeding to step (8). The lower limit for counter is
preferably set at zero, so the count cannot be decremented below
zero.
The interaction and implementation of the activation/inactivation
of the WLM, expiration/activation of the timer,
incrementing/decrementing and constraints associated with the
counter are implemented in control logic found in the controller.
During the time intervals where WLM is active, the counter
continues to increment. Utilizing the hysteresis allows the WLM
timer to be active until the number of timer counts reaches the
count value C.sub.MAX. Once the count reaches C.sub.MAX, then the
timer is expired and no longer increments the count and the WLM is
inactivated. WLM may also be inactivated by virtue of the WLM ratio
changing so that it is no longer greater than 0 and less than 1
(i.e. changes in the vehicle output torque command or output
speed). When the timer has expired, or WLM is no longer active
(whether due to the fact that the timer has expired or that the WLM
ratio has changed), the counter begins to decrement. The timer
remains expired and therefore WLM remain inactive, until the
counter value drops below the count value C.sub.DB. At which point,
the timer is active again allowing the counter to begin to
increment if the WLM is activated, or to continue to decrement to
zero if the WLM is not activated. The maximum value of the WLM
timer counter is limited by the operation of the timer as the
counter is only incremented if the timer has not expired and an
expired timer is defined as the counter value being greater than or
equal to the limit value C.sub.MAX. The minimum value of the
counter, however, is not limited in the same fashion. To keep the
counter from continuing to be decremented below zero, logic is
included to limit the counter to positive values and zero. This
method limits the amount of time the system can remain in WLM and
provides additional protection to the ESS by not allowing the
discharge limits to be expanded for extended periods of time.
Referring to FIG. 6, the operation and interaction of timer,
counter and WLM are illustrated and can be understood by reference
to the operation of WLM control algorithm 10 for a plurality of
control loops corresponding to time intervals 1-14. Interval 1
comprises a plurality of discreet time intervals associated with a
plurality of control loops as the WLM is activated, the timer
begins to count, and the counter is incremented 70 during each of
the subsequent control loops. The timer is expired and WLM is
inactivated when the counter reaches a preselected maximum count
value (C.sub.MAX). Interval 1 illustrates the maximum on time of
the WLM, which may vary from application to application depending
on the vehicle requirements, battery characteristics and other
factors, but will generally be in a range off 10-15 seconds. The
maximum on time should preferably be selected so as to ensure that
the parametric battery discharge limits are not exceeded, or if
exceeded due to overriding vehicle performance constraints, that
the duration during which a parametric discharge limit is exceeded
is minimized. Referring to interval 2, once the timer is expired,
the counter is decremented 90 during subsequent control loops until
the count is reduced to a value that is less than or equal to a
deadband value of the count (C.sub.DB), whereupon the timer is no
longer expired and the count may be incremented again once the WLM
is activated again. The deadband value of the count incorporates a
hysteresis into the count and the combination of C.sub.MAX and
C.sub.DB, together with the control logic work together to define a
duty cycle, such that the WLM cannot be active indefinitely, and
that the WLM is limited with regard to activation/inactivation so
that battery discharge limits established by the ESS or other
controllers are not exceeded sufficiently by shifting various
parametric limits to cause damage to the ESS. Referring again to
FIG. 6, the timer continues to decrement during interval 3 because
WLM is not active due to the WLM ratio during this interval. When
counter reaches zero, the WLM is still not active, the decrement is
limited so that the count is not less than zero 100, but the timer
is not expired. Referring to interval 4, the WLM remains inactive
and the count is still limited at zero, however, the timer is not
expired. In interval 5, the WLM is activated and the timer is not
expired, such that the count is incremented 70 until it reaches
C.sub.MAX, whereupon the timer is expired and consequently, WLM is
inactivated. Referring to interval 6, the timer is expired, WLM is
inactive and the counter is decremented even though the WLM becomes
activated because the count has not reached C.sub.DB. Referring to
interval 7, WLM is active, and because the count has reached
C.sub.DB timer is once again active and not expired, and count is
once again incremented until reaching C.sub.MAX. Referring to
interval 8, upon reaching C.sub.MAX, timer is expired and count is
decremented until the count reached C.sub.DB. Referring to interval
9, upon reaching C.sub.DB, timer is active or not expired, but the
count continues to decrement because the WLM is not active.
Referring to interval 10, the WLM is activated and the timer is not
expired, thus the count is incremented. Referring to interval 11,
timer is not expired, however, the WLM is inactivated and thus the
count is decremented. Referring to interval 12, the timer is still
not expired and the WLM is active, therefore, the count is
incremented. In interval 13, upon reaching C.sub.MAX, WLM is
inactivated, the timer is expired, and the count is decremented.
Referring to interval 14, the WLM is activated, however, the timer
is expired and the count is decremented until the count reached
C.sub.DB, whereupon the timer is not expired and the count is again
incremented.
Referring to FIGS. 1 and 7, the method 10 also comprises step (8)
of determining the amount of a WLM discharge power limit shift that
is based on the WLM ratio. The WLM is used to expand battery
discharge power limits that are used to protect the battery from
damage due to excessive discharge. The battery discharge power
limits are parametric limits in that they are limits that are
imposed based on various battery parameters that may be monitored
in order to ensure that the amount of energy discharged from the
battery at any given time interval (hence the term battery power
limits) does not damage the battery, including causing
disproportionate reductions in the battery service life. These
could include any discharge power limit associated with a parameter
that may be monitored so as to provide such protection to the
battery, but preferably include limits associated with the state of
charge (SOC) with regard to both the charging and discharging of
the battery, amp hour throughput and temperature. These limits may
be determined either empirically or theoretically for a given
parameter and in the case of computer control algorithm 10 are
preferably incorporated into lookup tables. In this way, a
calculated or measured value of the particular limit parameter may
be used in conjunction with a corresponding lookup table to
establish the particular battery power limit as a function of the
parametric value. Therefore, the method of the invention requires
that the parameters associated with the ESS, or battery, be
monitored and available as inputs for determination of the
parametric power limits. For example, if the battery temperature
during a vehicle launch is above a temperature where the maximum
discharge power limit would normally be reduced or derated, the
battery discharge power limit would normally be reduced based on
this temperature. Applicants have observed that this temperature
does not rise significantly and the battery is not damaged if this
derated limit value is exceeded for short periods of time, such as
during the launching of the vehicle. However, by allowing this
battery discharge power limit to be temporarily exceeded, the
vehicle performance can be greatly enhanced. Rather than simply
allow a complete violation of the battery discharge power limits
though, this method provides a means to shift or expand the limits
in a controlled manner and still provide the necessary discharge
power protections to the battery system.
The shifting of the parametric value is accomplished by shifting
the value of the parameter used as an input to the lookup table.
For example, in the case of a discharge power limit based on
battery temperature, the limit is derived from a lookup table as
shown in FIG. 7. The actual temperature is shifted to a lower
temperature, resulting in a shifted temperature based power
discharge limit such that a higher battery discharge power will be
allowed for the same actual battery temperature. In this example, a
batter power shift, T.sub.SHIFT, is subtracted from the actual
battery temperature to get the wide limit mode temperature:
The WLM temperature, T.sub.WLM, is then used as the input to the
lookup tables to determine the temperature based battery power
limit. If the actual temperature were used, the battery power limit
would be, P.sub.T.sub..sub.ACTUAL , and when the WLM temperature is
used the battery power limit would be P.sub.T.sub..sub.WLM , which
must be greater than, P.sub.T.sub..sub.ACTUAL , due to the downward
sloping nature of the battery discharge power limit lookup table
curve.
Similar shifts are applied to the state of charge (SOC), both with
respect to charging and discharging, and amp-hour/hour throughput
values such that the battery power limits based on each of these
parameters is expanded to a larger value when the WLM mode is
active. The amount of the parameter shift is preferably a fixed
amount, but may be selected so as to vary in magnitude as a
function of the magnitude of the parametric value with which it is
associated. In one embodiment, the temperature shift was about
5.degree. F., the SOC (charging) limit shift was about 10%, the SOC
(discharging) limit shift was about 10%, and the amp-hour/hour
shift was about 5 amp-hours/hour.
The magnitude of each of the parameter shifts are also adjusted as
a function of the WLM ratio, so that the amount of the shift is a
function of the commanded output torque and speed. These
adjustments are determined by multiplying the parameter shift by
the WLM ratio such that when the WLM ratio is 1, full shifting will
occur, and when the WLM ratio is 0, no shifting will occur, as
shown in FIG. 8. Once the shifted parametric discharge limits are
determined, the limiting or minimum shifted (WLM) parametric
discharge power limit is selected and output to the controller for
use during that control loop as the WLM discharge power limit.
For example, if the battery temperature during a vehicle launch is
above a temperature where the maximum discharge power limit would
normally be derated, the battery power limit will be reduced based
on this temperature. The battery temperature can not rise
significantly if this limit value is exceeded for short periods of
time, such as during the launching of the vehicle. However, by
allowing this limit to be exceeded, the vehicle performance can be
greatly enhanced. Rather than simply allow violation of limits,
this method provides a means to expand the limits in a control
manner and still provide the necessary protection to the battery
system.
Based on the foregoing, the invention may also be described
generally as a method of implementing a wide limit mode (WLM) of
operation in a vehicle comprising an energy storage system having a
rechargeable battery, the battery having a plurality of monitored
battery parameters, a discharge power limit and a closed-loop
controller, the controller having a timer and a counter that are
adapted to count time intervals associated with the implementation
of the WLM by incrementing a count when the WLM is active and
decrementing the count or maintaining a zero count when the WLM is
not active, comprising the steps of: (1) determining whether the
WLM is active; (2) setting a WLM discharge power limit when the WLM
is active that is greater than the discharge power limit; and (3)
establishing a duty cycle for the WLM using the timer and counter,
wherein the duty cycle comprises a maximum time interval during
which the increased discharge power associated with the WLM is
available for use by the vehicle and a minimum time interval during
which the increased discharge power associated with the WLM is not
available for use by the vehicle.
The foregoing discussion discloses and describes exemplary
embodiments of the present invention. One skilled in the art will
readily recognize from such discussion, and from the accompanying
drawings and claims that various changes, modifications and
variations can be made therein without departing from the true
spirit and fair scope of the invention as defined by the following
claims.
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